Glucagon-like peptide 1 receptor (GLP-1R) belongs to Family B1 of the seven-transmembrane G protein-coupled receptors, and its natural agonist ligand is the peptide hormone glucagon-like peptide-1 (GLP-1). GLP-1 is a peptide hormone arising by its alternative enzymatic cleavage from proglucagon, the prohormone precursor for GLP-1, which is highly expressed in enteroendocrine cells of the intestine, the alpha cells of the endocrine pancreas (islets of Langerhans), and the brain (Kieffer T. J. and Habener, J. F. Endocrin. Rev. 20:876-913 (1999); Drucker, D. J., Endocrinology 142:521-7 (2001); Holst, J. J., Diabetes Metab. Res. Rev. 18:430-41 (2002)). The initial actions of GLP-1 observed were on the insulin-producing cells of the islets, where it stimulates glucose-dependent insulin secretion. Subsequently, multiple additional antidiabetogenic actions of GLP-1 were discovered including the stimulation of the growth and inhibition of the apoptosis of pancreatic beta cells (Drucker, D. J., Endocrinology 144:5145-8 (2003); Holz, G. G. and Chepurny O. G., Curr. Med. Chem. 10:2471-83 (2003); List, J. F. and Habener, J. F., Am. J. Physiol. Endocrinol. Metab. 286:E875-81 (2004)).
Like GLP-1, Oxyntomodulin is also generated from L-cell derived proglucagon by alternative proteolysis. Oxyntomodulin is identical to glucagon plus an additional 8 amino acid carboxyterminal extension (Bataille D., et al, Peptides 2 Suppl s:41-4 (1981)). Oxyntomodulin is a dual agonist of both GLP-1 receptor and glucagon receptor. Oxyntomodulin induces glucose dependent insulin secretion from pancreatic β cells (Maida, A., et al, Endocrinology 149:5670-8 (2008), and in vivo, oxyntomodulin modulates food intake (Dakin, C. L. et al, Endocrinology 142:4244-50 (2001)) and is significantly anorectic (Baggio, L. L. et al, Gastroenterology 127:46-58 (2004)).
On activation, GLP-1 receptors couple to the α-subunit of G protein, with subsequent activation of adenylate cyclase and increase of cAMP levels, thereby potentiating glucose-stimulated insulin secretion. Therefore, GLP-1 is an attractive therapeutic target to lower blood glucose and preserve the β-cells of the pancreas of diabetic patients. Glucagon has been used for decades in medical practice within diabetes and several glucagon-like peptides are being developed for various therapeutic indications. GLP-1 analogs and derivatives are being developed for the treatment for patients suffering from diabetes.